Conventionally, retroreflective materials that retroreflect incident light are widely used for indications such as traffic signs, or for identification of marine accident equipment, and particularly for enhancing visibility during the nighttime. From the viewpoint of ensuring the safety of people who work at night, such retroreflective materials are also widely used as safety clothing for policemen, firefighters, workers involved in civil engineering and construction, and the like, in safety clothes, safety vests, sashes, arm bands, life vests, and the like. Further, in recent years, along with a growing consciousness of the safety of life, or the diversification of decorativeness, such retroreflective materials are also used in apparel such as windbreakers, sweat suits, T-shirts, sports shoes, and swimming suits as measures for preventing traffic accidents during the nighttime, or used in bags, suitcases, and the like for decorative purposes.
A typical retroreflective material has a structure in which transparent microspheres are additionally provided on a reflective layer, whereby light incident through the transparent microspheres is reflected at the reflective layer, and light is emitted through the transparent microspheres, so that light is retroreflected. In the retroreflective material with such a structure, a transparent resin layer may be provided between the reflective layer and the transparent microspheres to adjust the reflective luminance or the color tone of the reflected light. Conventional retroreflective materials are broadly classified into the three types, i.e., an open type, a closed type, and an encapsulated type, in accordance with the manner in which the transparent microspheres are embedded. In an open-type retroreflective material, a portion of the transparent microspheres are exposed in the air (see, for example, Patent Literature 1). In a closed-type retroreflective material, surfaces of the transparent microspheres (surfaces positioned opposite to the surfaces facing the reflective layer) are covered with a resin layer (see, for example, Patent Literature 2). In an encapsulated-type retroreflective material, there is space over the surfaces of the transparent microspheres (surfaces positioned opposite to the surfaces facing the reflective layer), and a resin layer is present over that space (see, for example, Patent Literature 3). Among these types, open-type retroreflective materials find wide application in the field of clothing, because they have high reflective luminance as well as flexibility.
On the other hand, while the conventional open-type retroreflective materials exhibit sufficient reflective performance for light incident from the front (i.e., incident light with a small incident angle), they have the drawback of having low reflective performance for light incident in an oblique direction (i.e., incident light with a large incident angle), which reduces the reflective luminance to result in poor visibility.
In recent years, an endless list of demands exists for enhancing the safety and decorativeness. In order to meet these demands, there is an earnest desire for the development of a technology of enhancing the retroreflective performance for incident light with a wide incident angle in open-type retroreflective materials.